Magnetic disc drives require the read/write head to track very accurately a very narrow data track on the disc while the disc surface passes under the head at very high speed. The head is kept centered on the track by a closed-loop servo system in which deviations of the head from the track center are detected by an off-track error detector (e.g. the detector disclosed in my copending application Ser. No. 735,851, filed Oct. 26, 1976, and entitled METHOD AND MEANS FOR CAPTURING MAGNETIC TRACKS, whose output is used as a position feedback to move the head back toward the track center). Stability of the head about the track center is achieved by detecting the velocity of the head motion by means of a tachometer and using the velocity indication as a damping velocity feedback. The resulting system is mathematically a damped second-order system.
Under the stringent operating conditions of a practical device, the head is constantly subjected to disturbance forces such as windage, vibration, or friction. The degree to which the head servo system is immune to such disturbance forces is known as the "stiffness factor" of the system, wherein the term "stiffness factor" has been defined generally as the angular lag between the input and output of a servo system. In the context of the present invention, the stiffness factor decreases as the immunity to the above-mentioned disturbance forces increases. In a conventional damped second-order servo system, the stiffness factor thereof is determined by such factors as the bandwidth, inertia (or mass) and damping of the system.
The natural frequency of a practical disc drive position servo must be limited to avoid excitation of mechanical resonances. Where economic factors dictate the use of a low-power actuator, the rotational inertia of the mechanism is limited to a rather low value. The use of such a very low inertia rotational position assembly, in conjunction with the limited bandwidth inherently allowable in mechanical systems, increases the stiffness factor in an appropriately damped stable second order system, to an intolerable degree.
Prior art disc drives did not have a stiffness factor limitation problem because they used linear positioners with inherently high mass requiring very high power inputs. Having paid the price of a high power position actuator, a suitable stiffness factor was usually allowed by the mass of the actuator and load. In addition, the prior art in some instances used two special techniques which should not be confused with this invention because they do not achieve the same result. In the first of these, the time integral of position error was fed back into the system. This results in a third order system with a decreased stiffness factor at low frequencies; however, the desired stiffness factor cannot be provided up to the natural frequency of the second order system, due to stability requirements. The second prior art technique used current feedback from the position actuator, which is related to acceleration. This technique can reduce the effect of the actuator time constant, but does not achieve a decreased stiffness factor.